In recent years, GaN compound semiconductor material has received much attention as semiconductor material used for short wavelength light emitting devices. A GaN compound semiconductor is formed on an oxide substrate such as a sapphire single crystal substrate, or Group III-V compound substrates by a metalorganic chemical vapor deposition method (MOCVD method) or a molecular beam epitaxy method (MBE method).
A sapphire single crystal substrate has a lattice constant which differs from the lattice constant of GaN by 10% or more. However, since a nitride semiconductor having excellent properties can be formed by forming on a sapphire single crystal substrate, a buffer layer comprising MN or AlGaN, a sapphire single crystal substrate is widely used. For example, when a sapphire single crystal substrate is used, an n-type semiconductor layer, a light emitting layer, and a p-type semiconductor layer are formed on the sapphire single crystal substrate in this order. Since a sapphire single crystal substrate is insulant, in general, in a device comprising a sapphire single crystal substrate, both a positive electrode formed on the p-type semiconductor layer and a negative electrode formed on an n-type semiconductor layer are positioned on one side of the device. Examples of a method for extracting light from a device comprising positive and negative electrodes on one side includes a face-up method in which light is extracted from the p-type semiconductor side using a transparent electrode such as ITO as a positive electrode, and a flip-chip method in which light is extracted from the sapphire substrate side using a highly reflective film such as Ag as a positive electrode.
As is explained above, sapphire single crystal substrates are widely used. However, since sapphire is insulant, a sapphire single crystal substrate has some problems. First of all, in order to form the negative electrode, the n-type semiconductor is exposed by etching the light emitting layer; therefore, the area of light emitting layer is reduced by the area of the negative electrode, and output power decreases. Secondly, since the positive electrode and the negative electrode are positioned on the same side, electrical current flows horizontally, current density is increased locally, and the device generates heat. Thirdly, since heat conductivity of a sapphire substrate is low, generated heat is not diffused, and the temperature of the device increases.
In order to solve these problems, a method is used in which a conductive substrate is attached to a device comprising an n-type semiconductor layer, a light emitting layer, and a p-type semiconductor layer which are stacked on a sapphire single crystal substrate in this order, the sapphire single crystal substrate is removed, and then a positive electrode and a negative electrode are positioned on both sides of the resulting stacked layers (For example, Japanese Patent (Granted) Publication No 3511970).
In addition, the conductive substrate is formed by plating, not by attaching (For example, Japanese Unexamined Patent Application, First Publication 200-47704).